(205d) Comparison between Formic Acid Decomposition and CO Oxidation with Au Catalysts

Traditionally, gold has been envisioned a very poor active catalyst for formic acid decomposition because large Au particles display a high activation energy barrier for HCOOH adsorption. In this work, we have found that small Au particles (< 5 nm) are, however, much more active than Pt in the formation of CO2 and H2 from HCOOH. The reaction kinetics (zero-order in HCOOH partial pressure) and isotopic studies strongly suggest that formic acid decomposes on Au particles via a bimolecular reaction mechanism involving the participation of formate species and adsorbed HCOOH molecules.

The size of the gold particles did not change when the catalyst was treated under O2/He at different temperatures. The TEM images reveal that the Au particle size did not increase considerably as a result of the thermal treatment. However, the HCOOH decomposition rate decreased gradually as the treatment temperature was increased. Since gold particles sintering was not observed by TEM, our results suggest that very small (not detectable by TEM) Au species constitute the active sites. On the other hand, we have observed that the rate of CO oxidation in the presence of water is not affected significantly by the thermal treatment of the catalyst. In this case, our results suggest, therefore, that the active sites for the CO oxidation reaction correspond to the metallic Au particles (3-4 nm) observed by TEM.

Finally, we have also carried out a mechanistic study of the CO oxidation reaction with the Au catalyst. In particular, we have focused on the role of water presence and concentration on the reaction kinetics and mechanism. We have found that water presents is absolutely needed in order to attain a high activity and stability. Furthermore, the water effect depends on the partial pressure. These results highlight that a carefully control of water concentration is absolutely needed when testing the Au catalysts. In fact, this could be, at least partially, the reason of the many conflicting results that can be found in the literature about the water effect in the CO oxidation with Au-based catalysts.

Acknowledgements

L. Rendon, F. Ruiz and M. Avalos from ?Centro de Ciencias de la Materia Condensada, UNAM, Mexico? are grateful acknowledged for the TEM images.